The long-term objective of this research project is to identify the neural circuit that is responsible for the generation of light-elicited ciliary locomotion and reflex-elicited foot contraction; and to understand how Pavlovian conditioning changes synaptic connections between neurons and alters intrinsic cellular excitability in the circuit. The experiments proposed in this grant will provide insights into how a neural circuit can be modified by Pavlovian conditioning to support the generation of two conditioned responses; CS-elicited foot contraction and conditioned inhibition of light-elicited locomotion. To achieve these goals, a multidisciplinary approach to the study of the cellular and synaptic basis of light-elicited ciliary locomotion and foot contraction will be carried out in the marine mollusk Hermissenda; a preparation that has proven useful in biophysical, biochemical, and molecular studies of learning. One primary goal will be to analyze conductances that contribute to the enhanced intrinsic excitability of the interneurons of conditioned animals using voltage-clamp techniques. We will also further identify neurons in the circuit that supports the generation of light-elicited ciliary locomotion and reflex-elicited contraction of the posterior foot. Using isolated nervous systems, intact animals and semi-intact preparations, we will identify the neural elements of the circuit and determine how the properties of the network contribute to the generation of light-elicited ciliary locomotion and reflex-elicited foot contraction. A second major goal is to determine how the circuit supporting locomotion and reflex-elicited foot contraction can be modified by Pavlovian conditioning. We will examine ciliary activation and statocyst-elicited foot contraction in semi-intact preparations while recording from interneurons and pedal motor neurons. Interneurons that receive sensory input from identified photoreceptors and hair cells, and project to cilia-activating and foot contraction pedal motor neurons will be identified and characterized. We will examine synaptic input to polysensory type I interneurons of conditioned animals to determine how Pavlovian conditioning modifies one sensory input, mediated by the conditioned stimulus (CS), without increasing the responsiveness of the type I interneurons to other sensory inputs. This experimental approach will expand our understanding of conditioning in Hermissenda by identifying within the neural circuit supporting the generation of behavior, other potential loci of plasticity produced by conditioning and provide the database for understanding how intrinsic cellular and synaptic modifications specific to one modality can be expressed in a circuit that must also support normal polysensory processing. These studies will help to elucidate general principles underlying the organization of polysensory neural systems generating behavior and their modification by learning.